Inspiring examples of large, multi-day online gatherings

Hey folks! A question for those who geek out to the challenges of online event design.

Have you ever experienced wonderful well-facilitated online events? I would warmly welcome any inspiring examples that come to mind in which online discussion, webcasts, "unhangouts" or other forms of social media engagement were woven together in an engaging and effective way?

Why I'm asking -- I am organising a multi-day "virtual summit" to launch a major report and host some dialogue and discussion around its findings. My organization has one foot in academia and one foot in community work. It tends to favour more traditional event formats -- with some willingness to experiment and hold space for dialogue.

I would be much obliged for your links and leads in the comments below! :-)